Disease-carrying flies seek out cannabis

Sand flies, vectors of a parasitic disease called leishmaniasis that kills up to 30,000 people every year, have a strong attraction to cannabis, multi-country research reveals.

A team of scientists led by Ibrahim Abbasia from The Hebrew University of Jerusalem, Israel, collected sand flies from five regions around the world and sequenced plant material found in their guts.

The results indicated that the insects fed on a variety of plants, but marijuana (Cannabis sativa) was by far the most common. This was doubly surprising, first because of the consistency of the finding in places as distantly separated as Tubas in Palestine and Bahia in Brazil, and second because in all but one test site the plant was not visibly common.

“Presumably, because cultivation of C. sativa is illegal in the countries where we worked, we did not see Cannabis plants in any of the sampling sites except for Kazakhstan, where C. sativa shrubs grew endemically,” write the researchers in the journal Proceedings of the National Academy of Sciences (PNAS).

“Therefore, we conclude that Cannabis comprised but a small fraction of the available sugar sources in any particular habitat and that its ample representation among sand fly plant meals signifies bona fide attraction.”

There are several species of sand fly, all from the family Psychodidae, that transmit leishmaniasis as part of their life cycle. Typically, the parasite is transferred into the human bloodstream by female flies. The process is a two-way street, with insect and humans repeatedly infecting each other.

In addition to blood, however, both male and female sand flies need sugar derived from plants. They consume this by piercing stems and leaves to access sap. The digested products of this can, in turn, be retrieved by researchers and subjected to sequencing. In particular, identifying a specific target – the ribulose bisphosphate carboxylase large chain (rbcL) gene – can lead to accurate identification of the source plants.

Which, in the case of sand flies trapped in Palestine, Israel, Brazil, Kazakhstan and Ethiopia, turned out most commonly to be marijuana.

Abbasia and his colleagues suggest a couple of possible reasons to explain the preference, but concede immediately that they are merely speculative. Aside from the obvious sugar haul – available from almost any plant – they suggest that cannabinoids may work to reduce the flies’ own leishmaniasis infections.

They note that the insects do not possess cannabinoid receptors, so they are not deriving any psychoactive benefit from their diet.

The researchers note that one public health control method in use against the flies in some countries involves the deployment of toxic baits that attract the insects by mimicking the scent of food plants. They suggest that adding the aroma of cannabis may well increase their efficiency.